Method for improving the colour index of cellulose esters

ABSTRACT

The invention relates to a method for improving the colour index of cellulose esters especially consisting of cellulose acetate. According to the inventive method, a delignified eucalyptus pulp is subjected to a preliminary treatment in order to remove pigments or chromophoric substances, and the process-modified eucalyptus pulp obtained is converted into cellulose esters in a conventional manner. The inventive method is characterized in that a delignified eucalyptus pulp having a cellulose content of more than approximately 80 wt. % and a pentosan content of more than approximately 1 wt. % is extracted by means of a solvent which dissolves oligomer pentosans but not the pulp of the delignified eucalyptus pulp, especially at a temperature of up to approximately 200 ° C. and a pressure of up to approximately 30 bar. The inventive method leads to an improvement in the colour index of cellulose esters or the fibres and filaments produced therefrom, in an economical and technically simple manner.

The invention relates to a method for improving the color index ofcellulose esters, especially consisting of cellulose acetate, wherein adelignified eucalyptus pulp is subjected to a preliminary treatment inorder to remove pigments and/or chromophoric substances, and theprocess-modified eucalyptus pulp that is obtained is converted intocellulose esters in a conventional manner.

The invention is based upon the problem that in the further processingof sheets of delignified eucalyptus pulp to cellulose esters, especiallyto cellulose acetate, color problems can arise. Such problems emergeespecially when the cellulose esters are processed to form fibers orfilaments to be used in filter materials for cigarettes. The process ofusing acetic acid to activate pulp in order to support a subsequentacetylation is already known. However the pigments or chromophoricsubstances are not removed in this process. They remain in the systemand lead later to an undesirably high color index, for example in thecellulose ester fibers. EP 0 503 304 A1 describes a method for treatingraw materials containing cellulose with a pulping liquor comprised ofconcentrated aqueous acetic acid at an increased temperature andincreased pressure. This is intended first to delignify the raw materialcontaining the cellulose. This process is subject to relatively severelimitations. For instance, the aqueous acetic acid that is used musthave a water content of between 5 and 50 wt. %, the temperature must liebetween 140 and 230° C., and the pressure must lie between 3 and 30 bar.A bleaching with ozone is imperative. With this, pigments andchromophoric components are also bleached. They further lead in theesterification and the processing into fibers or filaments to theabove-mentioned color problems. A correlation exists between the colorindex of a given cellulose ester and the material that is spun intofibers or filaments.

The degree of purity of the cellulose ester that is obtained, especiallyof the cellulose acetate, plays a major role in special areas ofapplication for cellulose derivatives, e.g. filter tow, textiles orplastic compounds. In this, the above-mentioned color index is anindicator of the degree of purity. The prerequisite for fulfilling therequirement of a low color index in cellulose acetate is high brightnesslevels in the pulp that is used. Depending upon the process and the rawmaterial, especially when eucalyptus wood is used to produce the pulp,this requirement is not always fulfilled. The chromophoric componentssuch as pentosans, resins, waxes, and other resonant organic substancesthat are very difficult to identify remain in the pulp and underacetylation conditions can lead to a definite yellowing of the celluloseacetate. In the cellulose acetate process, the bleaching of chromophoriccomponents of this type is dependent upon potassium permanganate orhydrogen peroxide, for example. The use of eucalyptus pulps can thuslead to increased yellowing in the final product or to a poor colorindex. The color index is determined via an extinction of a 2 wt. %cellulose ester solution, especially a cellulose acetate solution, informic acid, measured in a cuvette having a path length of 10 mm by 420nm, and multiplied by the number 1000 (color index=extinction (420nm)×1000). Especially in the case of cellulose acetate, the color indexis a very good indicator of the brightness of the corresponding endproduct, e.g. a filter tow.

The object of the invention was to improve upon the method described atthe beginning such that both the cellulose ester that is obtained andthe fibers or filaments produced therefrom exhibit a desirableimprovement in the color index.

This object is attained in accordance with the invention in that in themethod described at the beginning a delignified eucalyptus pulp having acellulose content of more than approximately 80 wt. % and a pentosancontent of more than approximately 1 wt. % is extracted by means of asolvent which dissolves oligomer pentosans but not the pulp of thedelignified eucalyptus pulp, especially at a temperature of up toapproximately 200° C. and a pressure of up to approximately 30 bar.

The parent material for the method specified in the invention is adelignified eucalyptus pulp. The delignification of woods is the currentstate of the art. The process described above in connection with EP 0503 304 A1, for example, may be used for this. For the purposes of theinvention, the pulp from eucalyptus wood raw material, which is renewedvery quickly, can be used, and this pulp has an advantage over otherpulps, such as coniferous and deciduous wood pulps. It must beadequately delignified.

The delignified pulp, supplied in the form of a pressed material isfirst expediently processed into fibers using customary apparatus, ifnecessary with the addition of small quantities of customary additives,such as sulphuric acid, and preferably is held for a certain amount oftime at room temperature, up to an increased temperature (up to 140°C.). The desired result is obtained especially when a delignifiedeucalyptus pulp having a cellulose content of more than 80 wt. %,especially more than approximately 88 wt. %, is used. The pentosancontent may also lie above approximately 1 wt. %, especially betweenapproximately 1.5 and 3 wt. %. If the pentosan content is under 1 wt. %,then there is generally already per se a satisfactory color index in thecellulose esters or fibers and filaments produced therefrom. Thepreferred pentosan content ranges from approximately 1.5 to 3 wt. %. Ifthe level of 3 wt. % is exceeded, then in addition to a significantcoloration, other disruptive effects can emerge in the processing tocellulose esters, such as low filtration values or increased opacitiesin the reaction solutions.

If the delignified eucalyptus pulp has a very high and disadvantageousmoisture content, for example on a magnitude of approximately 7 toapproximately 12 wt. %, it is generally advantageous for this parentmaterial to be subjected to preliminary drying. In some cases it canalso be beneficial to perform a degasification prior to the extraction.This can be accomplished, for example, via a customary acetic acidvacuum evaporation.

In selecting the extraction material, it is necessary for the specialistin the field to consider only that the solvent that is used mustdissolve the oligomeric pentosans, especially xylan, but must not ormust only insignificantly dissolve the delignified eucalyptus pulp. Itis easy for a specialist in the field to find suitable solvents thatwill fulfill these framework conditions. These may include, for example,acetone, and especially carboxylic acids, preferably C₁-C₄ aliphaticmonocarboxylic acids or mixtures of these. Of particular advantage isthe use of highly concentrated aqueous acetic acid, especially having acontent of approximately 80 to 99 wt. % acetic acid.

If the above-described solvents are used, not only are the oligomericpentosans extracted, which is a primary goal of the invention, but allother disruptive or chromophoric substances, including hemicelluloses,waxes, and resins, are also extracted.

The further process conditions, such as temperature, pressure andprocess duration, are not critical. It is preferred that the temperatureduring extraction under standard pressure be between approximately 15and 30° C., and the duration of the extraction be between approximately5 minutes and 2 hours, especially between approximately 10 and 60minutes.

The nature of the extraction also is not critical for the invention.Thus the delignified eucalyptus pulp is extracted in a conventionalextractor, wherein it is beneficial for the extraction to be carried outin a continuous countercurrent. In detail, the process may be asfollows: The pulp is formed into fibers, e.g. in a liquor ratio ofapproximately 1:20 with acetic acid in a mixing vat, after which it ispressed off via a press to a residual content of approximately 50 wt. %acetic acid, and is placed on a conveyor belt that is permeable toacetic acid. Based upon the length of the conveyor belt, a desirablenumber of extraction steps in the countercurrent principle can berealized. In any extraction step an inorganic acid, e.g. sulphuric acid,may be added. However, the extraction steps may also be realizedcontinuously, discontinuously, or quasi-continuously using screwpresses. The extraction is expediently conducted until the pentosans orthe disruptive chromophoric substances, and especially resinouscompounds, are removed to such an extent that the cellulose esters thatultimately are obtained by means of acetylation will possess a maximumcolor index of approximately 18, especially a maximum of approximately15.

The central focus of the method specified is to remove the pentosans andchromophoric compounds. In this, a specialist in the field willpreferably control the extraction in such a way that in addition to thepentosans especially resinous companion substances are also largelyextracted. To support this process it is expediently provided that astrong inorganic acid, especially sulphuric acid, is added to theextraction medium at the end of the extraction process. This ispreferably followed by a further extraction process lastingapproximately 5 to 15 minutes. The quantity of sulphuric acid relativeto delignified eucalyptus pulp (dry mass) amounts to approximately 1part by weight sulphuric acid or strong inorganic acid to approximately10 to 200 parts by weight delignified eucalyptus pulp (dry substance).

For the further reaction to esters, it has proven advantageous for theprocess-modified eucalyptus pulp to be treated in a tempering device forapproximately 5 to 45 minutes, especially approximately 10 to 30minutes, at a temperature of approximately 25 to 50° C., especiallyapproximately 30 to 35° C. The tempering device may have the followingconstruction: A mixing vat, which is provided with a double mantel, forexample, allowing it to heat the contents of the vat to the desiredtemperature by means of a heat exchanger liquid. Alternatively, however,an insulated mixing vat may be used, if the extracting agent is heatedto the desired temperature prior to filling, for example via a heatexchanger.

The principal goal of the process is for the process-modified eucalyptuspulp to be converted to a cellulose ester, especially cellulose acetate.In the latter case it is of particular advantage for the acetylation tobe accomplished using acetic anhydride. This is a customary process, inwhich preferably the pretreated, acetic acid-moist pulp, to the extentthat it is not already present in the extracting agent, is convertedfirst to cellulose triacetate using a catalyst, e.g. sulphuric acid, andthen using acetic anhydride. This is followed by the establishment of aprocess-dependent water content level, and then controlled hydrolysis tocellulose acetate. The acetyl value can be adjusted via the conditionsof hydrolysis.

The product of the process in the form of a cellulose ester, especiallyin the form of cellulose acetate, can be further processed viaconventional means to formed pieces, especially to fibers and filamentsthat are used in filter tow for filter cigarettes. This is accomplished,for example, by dissolving the cellulose acetate, depending upon theprocess, in a certain quantity of acetone, and following a subsequentfiltration, spinning it in a dry spinning process.

Upon completion of the extraction, excess extracting agent is removed,e.g. via pressing, and/or is exchanged in the countercurrent with pureextracting agent. A balanced surplus of the extracting agent that ischarged with the eluted components is treated in a separate process.Known processes, such as total hydrolysis, ultrafiltration,distillation, chromatography, or a combination of these for example, maybe used for this. The purified extracting agents are preferably recycledand reused for the process.

The advantages that are associated with the present invention may berepresented as follows: The inventive method is first economical andproduces no harmful effects on the environment. With a simple extractionof delignified eucalyptus pulps using, for example, acetic acid, thecontent of chromophoric components is significantly reduced. In this,water content levels in the acetic acid of up to 20% arenon-problematic, and even lead to further benefits. Thus the yellowingof cellulose acetate can be significantly reduced if theprocess-modified delignified eucalyptus pulp is subjected to theextraction specified in the invention, especially acetic acidextraction, prior to acetylation, after which the acylated or acetylatedpulp is reduced using customary techniques (pressing) to solids contentsof approximately 50%. The solvents used in the extraction, especiallyacetic acid, and the solvents that are pressed off, especiallycarboxylic acid, can be reused in the extraction process following apurification stage (for example distillation). In the acetylation, theextracting agent acetic acid may remain in the pulp, because in anacetylation process it is a component of the customary reaction medium.Here, the disadvantage of other extracting agents relative to aceticacid is that prior to acetylation they must be largely removed, whichrequires some expenditure. Particularly surprising is the followingcircumstance: When fibers or filaments are produced from celluloseacetate from an acetone solution using a dry spinning process,disadvantageous, chromophoric substances, including pentosans, cancontinue to appear in the end product. If, however, the extractionspecified in the invention is performed beforehand, for example withacetic acid as the extracting agent, then surprisingly this no longeroccurs. Instead, the product that is obtained fulfills the requirementswith respect to a low color index. Further, it has been determined thatwith only a simple extraction of delignified eucalyptus pulp usingacetic acid, the chromophoric components can be reduced. In this, awater content in the acetic acid of up to 20% causes no disruption.Depending upon the degree of purity of the delignified eucalyptus pulpthat is used, up to 1 wt. % of deep dark brown substances, and in somecases even more, is removed.

The invention will be described in greater detail below, with referenceto examples:

EXAMPLE

In each of four examples, 10 g of air-dried, delignified eucalyptus pulpwere used. The test samples A, B, C, and D described below wereprocessed, evaluated and/or compared.

A: Processed manually into fibers acetylated without further treatment,

B: Extracted as specified in the invention in 500 ml acetic acid at roomtemperature for 120 minutes, then filtered off,

C: Extracted as specified in the invention in 500 ml acetic acid at 50°C. for 120 minutes, then filtered off,

D: Extracted as specified in the invention in 500 ml acetic acid at 100°C. for 120 minutes, then filtered off.

The obtained process-modified eucalyptus pulp was subjected to anactivation as follows:

The extracted eucalyptus pulp was weighed into a 250 ml threaded flask.This was filled to volume with 100 ml glacial acetic acid. The flask wassealed and clamped inside an overhead mixer. The flask was allowed torotate from overhead for 3 days at approximately 20 rpm. On the fourthday, 50 ml of glacial acetic acid and 0.3 ml concentrated sulphuric acidwere added. It was allowed to rotate from overhead for another 2 hoursfor activation. The activation was followed by an acetylation. In thisthe process-modified eucalyptus pulp was transferred to a temperingvessel that had been heated to 35° C. Tempering took place for 15minutes. Then, with the addition of 50 ml acetic anhydride, theacetylation and at the same time the stirrer were started. During theacetylation, the viscosity of the reaction mixture was controlled. Thereaction mixture was drawn using a Peleus ball and a glass tube (innerdiameter 5 mm) up to a marker (20 cm from the end of the tube). ThePeleus ball was then removed and the reaction mixture was allowed toflow out of the glass tube, which was held in a vertical position. Inthis process the time required by the reaction mixture to flow outcompletely from the mark was noted. When only 30 seconds of this timewere left, the acetylation was interrupted by adding 15 ml 60-% aceticacid, which at the same time initiated the hydrolysis. The solution wasstirred for another 4-5 hours at 70° C. and, once the acetyl value ofapproximately 55.0 had been reached, the cellulose acetate wasprecipitated in water and dried. It can be processed using customarymethods into fibers or filaments, and these can be used in the filtertow for cigarette filters.

The determination of color index was accomplished as follows:

2 g dry cellulose acetate were dissolved in 100 ml formic acid. Thesolution was then centrifuged for 10 minutes at 3000 rpm, and decanted.The extinction of the solution was measured in a photometer at 420 nm.The color index is the extinction multiplied by the number 1000.

Results: Sample: Color Index A 32 B 18 C 15 D 18

The comparison shows that the processed, delignified eucalyptus pulpsproduce a markedly improved color index, i.e. the color index of theuntreated product is 32, while the products treated as specified in theinvention lead to color indexes of 18, 15, and 18. A color index of 18or less easily fulfills the requirements of the subsequent processor,especially in the case of further processing to filter tow.

1. Method for improving the color index of cellulose esters, especiallycomprised of cellulose acetate, wherein a delignified eucalyptus pulp issubjected to a preliminary treatment in order to remove pigments and/orchromophoric substances, and the process-modified eucalyptus pulpobtained is converted into cellulose esters in a conventional manner,characterized in that a delignified eucalyptus pulp having a cellulosecontent of more than approximately 80 wt. % and a pentosan content ofmore than approximately 1 wt. % is extracted by means of a solvent thatdissolves oligomeric pentosans but not the pulp of the delignifiedeucalyptus pulp, especially at a temperature of up to approximately 200°C. and a pressure of up to approximately 30 bar.
 2. Method in accordancewith claim 1, characterized in that a delignified eucalyptus pulp havinga pentosan content of approximately 1.5 to 3 wt. % and/or a cellulosecontent of more than approximately 88 wt. % is extracted.
 3. Method inaccordance with claim 1 or 2, characterized in that the extraction iscarried out using a carboxylic acid, especially a C₁-C₄ aliphaticmonocarboxylic acid or a mixture of these.
 4. Method in accordance withclaim 3, characterized in that the extraction is carried out usinghighly concentrated aqueous acetic acid, especially having a content ofapproximately 80 to 99 wt. % acetic acid.
 5. Method in accordance withat least one of the preceding claims, characterized in that theextraction is carried out at a temperature of approximately 15 to 30° C.6. Method in accordance with one of the preceding claims, characterizedin that the extraction is carried out at a pressure of approximately 1bar.
 7. Method in accordance with at least one of the preceding claims,characterized in that the extraction is carried out over a period ofapproximately 5 minutes to 2 hours, especially approximately 10 to 15minutes.
 8. Method in accordance with at least one of the precedingclaims, characterized in that the extraction is conducted in acontinuous countercurrent.
 9. Method in accordance with at least one ofthe preceding claims, characterized in that the delignified eucalyptuspulp is subjected to a preliminary drying.
 10. Method in accordance withat least one of the preceding claims, characterized in that prior toextraction the delignified eucalyptus pulp is subjected to thedegasification of an acetic acid evaporation.
 11. Method in accordancewith at least one of the preceding claims, characterized in that astrong inorganic acid, especially sulphuric acid, is added to theextraction medium at the end of the extraction, after which extractionis continued another 5 to 15 minutes.
 12. Method in accordance with atleast one of the preceding claims, characterized in that theprocess-modified eucalyptus pulp is treated in a tempering vessel over aperiod of approximately 5 to 45 minutes, at a temperature ofapproximately 25 to 50° C.
 13. Method in accordance with at least one ofthe preceding claims, characterized in that the extraction is controlledsuch that in addition to the pentosan, especially resinous companionproducts are largely removed.
 14. Method in accordance with at least oneof the preceding claims, characterized in that the process-modifiedeucalyptus pulp is converted to cellulose acetate, wherein theacetylation is accomplished using acetic anhydride.
 15. Method inaccordance with at least one of the preceding claims, characterized inthat the extraction is carried out until the cellulose ester that isobtained has a maximum color index of approximately 18, especially amaximum index of approximately
 15. 16. Method in accordance with atleast one of the preceding claims, characterized in that the obtainedcellulose ester, especially cellulose acetate, is further processedusing customary methods to form fibers or filaments.